In controlled source electromagnetic (“CSEM”) prospecting, the electric and magnetic fields measured by the receivers are then analyzed to determine the electrical resistivity of the earth structures (subsurface formations) beneath the surface or seafloor, because the resistivity, is known to be strongly related to the pore fluid type and saturation. See, for example, U.S. Pat. No. 6,603,313 to Srnka.
The bulk electrical resistivity of reservoirs is often increased substantially when hydrocarbons are present. The increase can be of the order of 100's to 1000's of percent. However, increased formation resistivity alone may not uniquely indicate hydrocarbons. For instance, carbonates, volcanics, and coals can also be highly resistive. Nevertheless, spatial correlation of high formation resistivity with potential traps imaged by seismic or seismic attribute data provides strong evidence of the presence of oil or gas and valuable information on their concentrations.
Recent CSEM surveys have shown that shallow resisitivity in the earth can mask the electromagnetic responses of resistive hydrocarbons that are buried more deeply in the earth (a false negative). Conversely, shallow resistivity can be mis-interpreted to indicate the presence of deeper reservoir resistivity (a false positive).
The conventional method of interpreting marine controlled-source electromagnetic (CSEM) data is to compare the observed electromagnetic response to a selected, reference experiment at a unique frequency (typically ¼ Hz). The reference experiment is supposed to represent the background resistivity; any differences seen between the observed data at other locations and the reference data are interpreted as resistivity anomalies (S. Ellingsrud et al., The Leading Edge 21, 972-982, October 2002). The frequency is chosen to produce an optimal response of resistivity anomalies at the reservoir depth. Unfortunately, this frequency is also sensitive to shallower anomalies and these shallower anomalies can hide (or be mistaken for) deeper anomalies.
For instance, FIG. 1 shows the resistivity anomalies from a synthetic marine CSEM survey example where a frequency of ¼ Hz was used with a background resistivity of 1 Ohm-m. The reference experiment is located at 4 in a geologic syncline where no resistivity anomaly is present. Anomalies are defined with respect to this reference. If the electromagnetic response recorded at a receiver is close to the data recorded at the reference receiver, a white dot is displayed at the receiver location. A blue dot means that the data look slightly more conductive than the reference and a cyan dot means that the data look slightly more resistive than the reference. Yellow to red to dark-red dots show an increasingly anomalous high resistive behavior with respect to the reference receiver. The prominent mostly red feature 1 on the anomaly map corresponds to a very shallow resistivity anomaly at 6 Ohm-m (channel filled with low-saturation gas). A deeper but still relatively shallow oil-field (40 Ohm-m anomaly) is visible at 2, but the deeper main field 3 is completely hidden by the shallow anomaly overprint.
It is well known to practitioners in the art that the depth of penetration of electromagnetic data depends on the frequency of the signal. The amplitude of the data is attenuated to 1/e (e is the base of natural logarithms) at a distance δ=503(R/f)1/2 where R is the resistivity in Ohm-m, f is the frequency in Hertz and δ is the skin depth in meters. High-frequency electromagnetic data is rapidly attenuated away from the source and is not sensitive to deep anomalies. Low-frequency data is less attenuated and can penetrate deeper. It is sensitive to both shallow and deep resistivity structure. See, for example, Keller, G. V. and Friscknecht, F. C., Electrical Methods in Geophysical Prospecting, Pergamon Press, 90-196 and 299-353 (1966); Olm, M. C., Electromagnetic Scale Model Study of the Dual frequency Differencing Technique: M.Sc. thesis, Colorado School of Mines, Pergamon Press, N.Y. (1981); Kaufmann, A. A. and Keller, G. V., Frequency and Transient Soundings, Elsevier, N.Y., XVII-XXI, 213-314, 411-450, 621-678 (1983); B. R Spies, Geophysics 54, 872-888 (1989); Zhdanov, M. S, and Keller, G. V., The Geoelectrical Methods in Geophysical Exploration, Elsevier, N.Y., 347-450, 585-674, 692-701 (1994). These sources are standard references to electromagnetism practitioners; however, they contain little about the art of CSEM exploration in a marine environment, and none of them teach how to determine the effects of shallower electrical resistivity structures on the electromagnetic responses of deeper resistivity targets in marine CSEM surveying. The present invention satisfies this need.